Unexpected voltage level with EXP1616R

[ToddYoung]

Hello there!

I'm using a T100MD888+ in conjunction with a couple of EXP1616R to switch on air conditioners. 24VAC outputs from various thermostats are fed into the EXP1616Rs and their relays are then used to switch on the fans, heaters, and coolers. The problem that I'm facing is as follow: As soon as I connect the 24VAC outputs from the thermostats into the EXP1616R the voltage levels at the inputs on the EXP1616R immediately become unstable. That is, voltages would drop to around 14VAC where they should be at 24VAC and voltages would rise to around 7VAC where they should be at ground potential. Needless to say, these unexpected voltages are murder on my ladder logics! Any insight anyone can offer on this mystery would be greatly appreciated.

Many thanks in advance.

Todd...

[support]

It sounds like a wiring issue. Each PLC input draws only about 5mA of current so it is unlikely that the loading factor to turn on the input would cause the output from the thermostats to drop from 24VAC to 14VAC.  It is even harder to imagine how the PLC inputs would rise to 7V when there is no voltage (0V) applied to their input. It is most likely some short circuit somewhere in your system.

You have to check through your wiring again. Do remember that each 8 input terminals of the EXP1616R shares one common terminal and these AC inputs are completely isolated from the digital power supply (24VDC or 5V). So make sure that you have not inadvertently shorted the AC voltage to the DC voltage which would create havoc to your electrical system.